Here’s an interesting fact: Apparently the criteria for whether to execute running attacks only take into account your non-zero movement states.
So when deciding whether a normal attack needs to be modified, the game asks this: Is the player’s most recent movement ‘running’ or ‘backstepping’? Then do a running attack. Is it ‘rolling’? Then do a rolling attack. Is it ‘walking’? Then do not modify.
When you stop moving from a sprint, your character does a ‘sudden stop’ movement that the game counts as ‘walking’, so any time you stop running without attacking, you should do a normal attack. However! When you’re blocking, doesn’t matter with what, the ‘sudden stop’ animation doesn’t occur after sprinting. This means that for a short time, stopping a sprint while blocking lets you do a running attack from a standstill, because the game never actually thinks to check whether you’re standing still before deciding to do a running attack. Neat, huh? The same goes for a jumping attack, but you can already do that from a standstill, so it’s less interesting.
TL;DR: Sprint, and while holding block, stop abruptly. For a few seconds your next normal attack will be a running attack.